1. Field of the Invention
The invention relates to an extruder die head, preferably a blown film head, comprising an internal cylindrical mandrel and a jacket, which envelops concentrically the mandrel. Between the mandrel and jacket an annular channel is formed that empties into a die slit. The invention also comprises at least one line, which empties into the annular channel in the area opposite the die slit and which feeds a melt.
2. Related Art
Blown film heads of this type exist. Usually the mandrel exhibits one or several helical groove(s), whose depth decreases in the direction of the die slit, starting from the feed channel(s) so that the polymer melt overflows more and more the webs, which define the channels, and assumes a uniform flow in the axial direction. The prior art extruder die heads exhibit the special problem that the mandrel on the other side of the beginning of the annular channel is fitted sealingly into a cylindrical borehole of the jacket. If, however, the extruder die head becomes warm due to the polymer melt, flowing through said extruder die head, the jacket expands more, on account of its larger diameter, than the central mandrel so that between the two of them a slit can form, into which penetrates the polymer melt that is fed in under pressure. Since the melt can accumulate in the area of this slit and because of its long residence time in the hot die head, where it can decompose and become brittle, particles of the deposited melt can be dragged along by the melt. Said particles become obvious in the extruded film tube or the inflated film bubble as the defective spots.
Therefore, the object of the invention is to provide an extruder die head of the class described in the introductory part, wherein slits, in which the polymer melt can accumulate, deposit and become brittle, are avoided between the central mandrel and the jacket, enveloping the mandrel.
The invention solves this problem in that the mandrel is made as one piece with a flange-like foot and is provided in its area, adjacent to the foot, with at least one borehole, which feeds in the melt, and that on the mandrel adjacent to the foot is shrunk a ring, which seals the slit between both, and that on the ring is mounted a cylindrical jacket and is connected sealingly to said jacket, which envelops the mandrel, forming an annular channel.
In the case of the extruder die head of the invention, the mandrel is reliably sealed with respect to the flange-like foot, which can be connected to the mandrel as one material piece, by means of the ring that is shrunk on. The ring is shrunk on the mandrel with such a pretension that even if the mandrel and the ring become warm, no slit, into which the melt can penetrate and become brittle in said slit, can form between the two. The cylindrical jacket is mounted on the ring and connected sealingly to it.
A preferred embodiment provides that the mandrel exhibits in its area adjacent to the foot one or several helical groove(s), which exhibit(s) a depth that decreases in the direction of the die slit and which is (are) closed in the direction of the foot. The boreholes, which feed in the melt, empty in the initial regions of the grooves in the vicinity of the foot. In this respect the shrunk-on ring encloses sealingly the groove-free end of the mandrel and its initial region that is provided with the grooves. In this embodiment the shrunk-on ring seals without a gap the groove edges in the foot region of the mandrel so that from the grooves the melt, fed into the initial region of the grooves, can enter into the annular space between the mandrel and the jacket. In this space the melt can assume increasingly and uniformly an axial flow.
Preferably the foot and the ring are provided with radial annular surfaces, which lie in a common plane and which can lie sealingly on each other and can be mounted together so as to seal.
Preferably the ring and the jacket are also provided with annular surfaces, which lie in a common radial plane and which lie sealingly side by side and are mounted together.
To fasten the jacket to the ring, the jacket can be mounted by intercalating the ring through an expansion screw connection with the annular surface of the foot.
If the extruder die head is a blown film head, boreholes or channels can be provided in the known manner between the channels, feeding in the melt, in order to feed in and exhaust in the known manner the blowing air.
These together with other objects and advantages which will become subsequently apparent reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout.